I suspect part of my problem is that I'm having a mental freeze on what the technical name for this is: But you see on http://maps.google.co.uk/ the bottom left has a small ruler indicating how big x Miles is at the current zoom level?
If I'm using google Maps API version 2, how do I add such a wonderful do-hickery widget thing to my website maps?
Thanks, James
Actually this is how you really get the scale to show:
var map = new google.maps.Map(document.getElementById('map'), { zoom: 10, scaleControl: true, center: new google.maps.LatLng(-33.92, 151.25), mapTypeId: google.maps.MapTypeId.ROADMAP }); From the API docs:
GScaleControl - a map scale 
I found that if you call the map as a function rather than as var and apply the scaleControl as suggested, the entire map fails to show. Still working on a solution that solves this part of the problem.
function initMap() { map = new google.maps.Map(document.getElementById('map'), { center: {lat: 15.5, lng: 121}, zoom: 10 }); 